Explanation:
Some auroras
can only be seen with a camera.
They are called sub-visual and are too faint to be seen with the unaided eye.
The reason is that the human eye
only accumulates light for a fraction of a second at a time,
while a camera shutter can be left open indefinitely.
When photographing an already picturesque scene above
Juneau,
Alaska,
USA,
a camera caught green sub-visual aurora near the horizon.
Auroras
are sparked by
energetic particles from the
Sun impacting the
magnetic environment
around the Earth.
Resultant energetic particles such as
electrons and
protons
rain down near the Earth's poles and impact the air.
The impacted
air molecules
temporarily lose electrons, and when
oxygen molecules
among them reacquire these electrons, they emit
green light.
Auroras
are known to have many
shapes and
colors.